The present invention relates to a light-emitting semiconductor device such as a light-emitting diode.
A conventional light-emitting diode, referred to below as an LED, is formed by selective diffusion of a p-type impurity into an n-type semiconductor substrate. FIG. 1 shows an example In which the semiconductor substrate 10 comprises an n-type gallium-arsenide (GaAs) substrate layer 11 and an n-type gallium-arsenide-phosphide (GaAsP) epitaxial layer 12. Selective diffusion of zinc (Zn) creates a p-type diffusion region 14. The device is covered with an insulating film 15 in which a window is opened to permit a p-electrode 16 to make contact with the diffusion region 14. An n-electrode 17 is formed on the underside of the device, in contact with the n-type GaAs substrate 11. When a forward voltage is applied between the p-electrode 16 and n-electrode 17, light is emitted by radiative recombination of electrons and holes in the vicinity of the pn junction formed between the p-type diffusion region 14 and the n-type GaAsP epitaxial layer 12.
Nonradiative recombination also occurs, especially near the surface of the device, due to crystal defects present near the surface.
FIG. 2 shows a variation of this device in which the semiconductor substrate 20 has three epitaxial layers: an n-type GaAs.sub.1-x P.sub.x graded buffer layer 21 in which the phosphorus concentration (the value of x) gradually increases in the upward direction; an n-type GaAs.sub.1-y P.sub.y layer 22 in which the phosphorus concentration (y) is constant; and an n-type GaAs.sub.1-z P.sub.z layer 23 in which the phosphorus concentration (z) is constant and is greater than the concentration (y) in the GaAs.sub.1-y P.sub.y layer 22. The p-type zinc diffusion 24 extends through the GaAs.sub.1-z P.sub.z layer 23 into the GaAs.sub.1-y P.sub.y layer 22. Because of the different phosphorus concentrations, light emitted in the GaAs.sub.1-y P.sub.y layer 22 is not absorbed in the GaAs.sub.1-z P.sub.z layer 23, so light emission is increased.
A problem in these conventional LEDs is that the p-type impurity (zinc) diffuses laterally as well as vertically, and near the surface of the device, the lateral diffusion is irregular. In the device in FIG. 2, in particular, the diffusion front becomes extremely irregular on a microscopic scale at the interface between the GaAs.sub.1-z P.sub.z layer 23 and the GaAs.sub.1-y P.sub.y layer 22. The irregularities create strong electric field pockets, so that when a forward voltage is applied, much of the forward current is channeled across the pn junction at points near the surface (in the GaAs.sub.1-z P.sub.z layer 23 in FIG. 2, for example), where nonradiative recombination is most likely to occur. The light-emitting efficiency of the device is thereby reduced.